Rare earth sintered magnets currently used extensively in various fields of applications include a samarium-cobalt (Sm—Co) type magnet and a neodymium-iron-boron type magnet (which will be herein referred to as an “R-T-(M)-B type magnet”). Among other things, the R-T-(M)-B type magnet (where R is at least one of the rare earth elements including yttrium (Y) and is typically neodymium (Nd), T is either Fe alone or a mixture of Fe, Co and/or Ni, M is at least one additive selected from the group consisting of Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta and W, and B is either boron alone or a mixture of boron and carbon), is used more and more often in various types of electronic appliances. This is because the R-T-(M)-B type magnet exhibits a maximum energy product (BH)max that is higher than any of various other types of magnets, and yet is relatively inexpensive.
A rare earth sintered magnet is produced by pulverizing a rare earth alloy into an alloy powder, pressing and compacting the alloy powder under a magnetic field to obtain a green compact (as-pressed compact) and then sintering the green compact in a sintering furnace. If the rare earth element such as neodymium to be included in the R-T-(M)-B type magnet is oxidized during the sintering process, the resultant magnetic properties deteriorate significantly. Thus, to avoid the disadvantageous oxidation, the atmosphere inside the sintering furnace is normally a vacuum or a reduced-pressure inert atmosphere of Ar, He, or any other inert gas. In sintering multiple green compacts, those green compacts are loaded into a hermetically sealable sintering case (which is also called a “sintering pack”) and then the sintering case, including those green compacts, is heated in its entirety to increase the productivity. Also, when a great number of green compacts are to be sintered simultaneously, a sintering case, equipped with a number of sintering base plates piled up like shelves, is used. In that case, the as-pressed green compacts are arranged on the sintering base plates and then those plates are stored like shelves inside the sintering case.
For example, green compacts 95 to be processed into sintered magnets for a motor are sintered after having been arranged inside a sintering case 9 as shown in FIGS. 3A and 3B.
In the example illustrated in FIGS. 3A and 3B, the sintering case 9 includes a bottom container 90 and a cover 92 to be fitted with the bottom container 90. The bottom container 90 includes a bottom plate 90a and a sidewall 90b. Inside the bottom container 90, a number of sintering base plates 94 are vertically piled up with a predetermined gap provided between them by spacers 96. The sintering case 9 is heated up to an elevated temperature of about 1,000° C. or more, for example, during the sintering process. Accordingly, the bottom container 90 and the cover 92 are both made of a material with high thermal resistance (e.g., molybdenum or SUS310).
The sidewall 90b of the bottom container 90 surrounds the periphery of the sintering base plates 94 and supports the cover 92 thereon at the upper edge thereof. The space surrounded with the sidewall 90b (i.e., storage space) has a horizontal dimension (i.e., a width) that is slightly greater than the width of the sintering base plates 94. The difference may be on the order of several millimeters to several centimeters. In any case, this sintering case 9 is designed so as to have a narrow gap between the sintering base plates 94 and the sidewall 90b. This narrow gap is adopted to store the greatest possible number of green compacts 95 inside the sintering case 9 simultaneously as efficiently as possible. This is because the narrower the gap, the greater the width of the sintering base plates 94 can be. In addition, when the gap between the sintering base plates 94 and the sidewall 90b is small, even if the sintering case 9 is vibrated during its transportation, for example, the sintering base plates 94 cannot move inside the sintering case 9 so much as to collapse the spacers 96 on the sintering base plates 94 unintentionally.
As shown in FIGS. 4A through 4C, each of the green compacts 95 has curved surfaces including a concave surface 95a and a convex surface 95b. When the green compact 95 shown in FIG. 4A is viewed along a plane that crosses the concave and convex surfaces 95a and 95b at right angles, the cross section of the compact 95 has a shape including two arcs. For example, the concave and convex surfaces 95a and 95b may constitute respective portions of two cylindrical surfaces having mutually different radii of curvature. In that case, the outer radius defined by the convex surface 95b may be greater than the inner radius defined by the concave surface 95a. A green compact having such a shape is called a “curved green compact” or an “arched green compact”. As shown in FIG. 4A, this green compact 95 includes two curved surfaces (i.e., the concave and convex surfaces 95a and 95b, which will be herein also referred to as “principal surfaces”) that are opposed to each other; two side surfaces 95d that are opposed to each other with the two curved surfaces 95a and 95b interposed between them; and two end surfaces 95c that cross both the curved surfaces 95a and 95b and the side surfaces 95d substantially at right angles. The principal surfaces 95a and 95b are greater in area than any of the other surfaces of the green compact 95. Typically, the end surfaces (or bottoms) 95c are smaller in area than any other surface of the green compact 95.
The green compacts 95 of such a shape are mounted on each of the sintering base plates 94 so as not to contact with each other, e.g., so that the horizontal edges of the concave surface 95a or the center of the convex surface 95b is in contact with the sintering base plate 94 as shown in FIGS. 4B and 4C. These arrangements are used to prevent the green compacts 95 from turning over in the manufacturing and processing step of mounting the green compacts 95 on the sintering base plate 94 or loading the sintering base plate 94 into the case 9, for example. For that purpose, the green compacts 95 are arranged to have their center of mass located at the lowest possible level (i.e., so that their top is located at the lowest possible level) when mounted on the sintering base plate 94. To increase the degree of orientation, the green compacts 95 (e.g., green compacts to be processed into R-T-(M)-B type magnets, in particular) have a green density that is lower than that of green compacts to be processed into ferrite magnets. The green compacts 95 to be processed into R-T-(M)-B magnets may have a green density of about 3.9 g/cm3 to about 5.0 g/cm3, for example. Accordingly, these green compacts 95 are very brittle and easily crack or chip on impact with something hard (e.g., the instant they fall or are dropped). Thus, these green compacts 95 should be arranged so as not to turn over so easily. It should be noted that the green compacts 95 arranged on the same sintering base plate 94 may have been either subjected to the compaction process individually or obtained by cutting and dividing a single green compact into multiple smaller bodies.
Furthermore, if the green compacts 95 that have been mounted directly on the sintering base plate 94 are sintered, then the resultant sintered bodies 95 and the sintering base plate 94 may sometimes be partially fused together unintentionally. This is because the rare earth element such as Nd included in the R-T-(M)-B type alloy powder and a metal element included in the sintering base plate 94 may cause a eutectic reaction at a temperature that is equal to or lower than the sintering temperature. If the base plate 94 and the sintered bodies 95 are partially fused together, the size of the green compacts 95 being sintered does not decrease smoothly with the sintering process, thus possibly cracking or chipping the resultant sintered bodies 95. Also, even if the base plate 94 and the sintered bodies 95 are not fused together, non-uniform friction may be created between the base plate 94 and the sintered bodies 95, thus also possibly cracking the sintered bodies 95 on their surface that is in contact with the sintering base plate 94.
Thus, to prevent the sintering base plate 94 and the sintered bodies 95 from being fused together, the surface of the sintering base plate 94 is coated with a bedding powder (not shown) according to a known technique so that the green compacts 95 can be sintered on the bedding powder (see, for example, Japanese Laid-Open Publication No. 4-154903). The bedding powder needs to be a powder of a material that exhibits low reactivity with the green compacts 95 and high chemical stability at an elevated temperature. When the green compacts 95 include a rare earth metal, the bedding powder may be a powder of a material exhibiting low reactivity with the rare earth metal, e.g., a powder of a rare earth oxide such as neodymium oxide or yttrium oxide. By using such a bedding powder, the sintering base plate 94 and the sintered bodies 95 are not fused together, and therefore, portions of the sintered bodies 95 that are in contact with the base plate 94 are neither damaged (e.g., cracked) nor deformed.
However, if multiple green compacts 95 are arranged inside the sintering case 9 as shown in FIGS. 3A and 3B, then the number of green compacts 95 that can be stored inside the sintering case 9 at the same time is relatively small, and the sintering process cannot be performed so efficiently. Specifically, when the flat-plate green compacts 95 are mounted so as to have their center of mass located at the lowest possible level, the projection area of each of those green compacts 95 on the base plate 94 is rather great, thus decreasing the number of green compacts 95 that can be arranged within a limited area. As used herein, the “projection area” of each green compact 95 means the area that is covered by the green compact 95 on the base plate 94.
Also, if the green compacts 95 are mounted as shown in FIG. 4B or 4C, each of these green compacts 95 is in contact with the base plate 94 in just a narrow area. Then, as the sintering process advances, the (frictional) stress that is created due to the shrinkage of the green compact 95 will be concentrated on the contact portions. In that case, even if the bedding powder is used as described above, the sintered body 95 is still damaged or deformed often by the frictional stress that is created.
Furthermore, when the green compact 95 is mounted as shown in FIG. 4C, portions located around the center of the convex surface 95b of the compact 95 are damaged or deformed. Thus, it is impossible to remove only the damaged or deformed portion of the sintered body 95 and use the remaining portion thereof. On the other hand, when the green compact 95 is mounted as shown in FIG. 4B, the concave surface 95a of the compact 95 has its horizontal edges deformed. This concave surface 95a has a shape that should not be deformed to fit the resultant sintered magnet on the rotor shaft of a motor. Accordingly, it is also difficult to remove only the deformed portions therefrom and process the remaining portion into a predetermined shape for a sintered magnet. That is to say, if any of the sintered bodies that have been mounted as shown in FIG. 4B or 4C becomes defective, then the defective sintered body cannot be used anymore, thus decreasing the yield of sintered magnets significantly.
On the other hand, Japanese Laid-Open Publication No. 61-125114 discloses a technique of reducing the number of defective (e.g., warped or deformed) sintered bodies in making relatively thin rare earth sintered magnets. According to the technique disclosed in Japanese Laid-Open Publication No. 61-125114, a green compact having a small thickness is sandwiched between a pair of thicker green compacts that is made of the same material, and has the same shape, as the former green compact. Also, according to the technique, a powder of a material that does not react with the green compacts easily is interposed between these green compacts and/or between the green compact and the base plate when needed.
In the method disclosed in Japanese Laid-Open Publication No. 61-125114, however, not only the thin green compact but also two other thicker green compacts should be prepared to obtain a single sintered body of the desired small thickness, thus decreasing the yield of the rare earth alloy powder material. Also, according to such a technique, it is difficult to increase the number of green compacts 95 that can be loaded into the sintering case 9 at the same time. Furthermore, in sintering the green compacts 95 having a shape such as that shown in FIG. 4A, it is difficult to sufficiently reduce the damage or deformation of the resultant sintered bodies 95 due to the frictional stress created by the shrinkage of the green compacts 95 being sintered. It is rather understandable that the frictional stress, which is created between the lowest one of the green compacts stacked and the base plate, would be increased to further damage or deform the resultant sintered body, because the total mass of the vertically stacked green compacts is applied to the lowest green compact that is in contact with the base plate.
As described above, the green compact of a rare earth alloy powder has a great specific gravity (e.g., a green compact of an R-T-(M)-B type alloy powder has a specific gravity of about 3.9 g/cm3 or more) and is very brittle. Accordingly, when a frictional stress is created due to the shrinkage of the green compact being sintered (which loses as much as about 40% or more of its volume), the sintered body is easily damaged or deformed. Particularly when a green compact is mounted so as to have its center of mass located at a low level and to have a small area of contact with the base plate as shown in FIG. 4B or 4C, the resultant sintered body is damaged or deformed very easily. In addition, it is also difficult to store such green compacts efficiently inside a sintering case.